Aviation, wireless industries clash over C-band interference

AT&T and Verizon are gearing up to deploy the first tranche of C-band spectrum for 5G later this year, but before they do, groups in the airline and aerospace industry say the carriers need to put short-term solutions in place to avoid harmful interference – or expect major disruptions to air travel, transport, and emergency helicopter operations.

However, a July white paper from industry group 5G Americas says studies submitted by the aviation industry have “significant shortcomings” and are overly conservative when it comes to real-world conditions.

C-band spectrum between 3.7-3.98 GHz was auctioned by the Federal Communications Commission, with Verizon and AT&T spending over $45 billion and $23 billion, respectively (plus billions more for clearing costs and payments). These mid-band frequencies are key for 5G coverage and capacity.

This month, ex parte (PDF) filings (first spotted by Light Reading) show that 19 groups and companies representing the aerospace and aviation industry met with FCC officials urging the agency to grant a pending petition to reconsider part of the 2020 C-band order and put carrier-initiated steps in place by December 5, 2021, “to ensure aviation and public safety by protecting radio altimeters from harmful interference from 3.7 GHz licensed operations.”

In its February 2020 Report and Order approving use of the C-band for wireless service, the FCC had concluded “well-designed [radio altimeter] equipment should not ordinarily receive any significant interference (let alone harmful interference) given these circumstances.”

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Radio altimeters are key safety-of-life systems used to measure the height of an aircraft above the ground, up to 2,500 feet depending on aircraft, where harmful interference could cause issues in various landings and approaches.

For example, the aviation groups recently told the FCC that “such harmful interference could lead to an escalation of negative outcomes, from missed approaches, delays, diversions, and flight cancellations, to the shutting down of runways on an indefinite basis.”

Concerns over interference to aviation systems, specifically to radio altimeters operating in the nearby 4.2.-4.4 GHz band, had been raised by industry groups before the auction, as well as by Chair of the House Committee on Transportation and Infrastructure Rep. Peter DeFazio, D-Oregon, earlier this year.

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Tim Farrar, president of consulting and research firm TMF Associates, told Fierce he thinks the FCC “was shocked that the aviation community objected at the last minute before the auction was to take place last December, when it had been known since 2017 that there was a high likelihood of spectrum being reallocated in the 3.7-4.2 GHz band.”

He said particularly after major contention over Ligado and its L-band spectrum “the FCC is very sensitive to attempts by other parties (especially when they secure support inside the executive or legislature) to try and exercise veto rights across large swathes of spectrum.”

The FCC released a Notice of Inquiry (NOI) in August 2017 well before the February 2020 C-band order, the latter which 5G Americas says explicitly took into account comments from the aviation community (including leaving a 220-megahertz guard band) which had been active in the proceeding.

Ahead of the December 2020 C-band auction, the Radio Technical Commission for Aeronautics (RTCA) provided an October analysis, which the aviation industry cited in recent filings and 5G Americas’ white paper refutes.

Pointing to the RTCA study, the airline groups this August reiterated their stance to the FCC that the 220-megahertz guard band wasn’t enough to protect radio altimeters, which are used on several types of civil aircraft worldwide. They fear interference given the high power levels that are allowed for 5G operations in the C-band.

Based on the report, “safe interference limits are exceeded by 5G fundamental emissions at up to 500 ft altitude” for commercial transport airplanes like large jet airlines, “and across the entire operational altitude range” of up to 2,500 feet for helicopters, general and business aviation, and regional transport airplanes.

5G Americas’ report asserts technical issues with the RTCA study, such as restrictive inputs and conditions, and concluded that “when evaluating real-world conditions, conclusions of the study would be different…” It also points to 5G deployments in Japan, South Korea and Europe.

“Several countries have been deploying 5G in spectrum near the radio altimeter band with no reports of interference. In the United States, the federal government has operated radar and communications systems in spectrum near the radio altimeter band for decades,” 5G Americas’ report states.

In presentations to FCC officials, airlines rejected comments by wireless industry group CTIA, which in April also said (PDF) there were significant flaws in the RTCA report. CTIA said the FCC was correct in determining that 5G services in C-band can operate without interference and the agency “should dismiss the aviation industry’s unsupported claims.”

Spurious emissions and short-term steps

With commercial C-band deployments just around the corner, why are concerns once again raised?

“The altimeters are separated by hundreds of megahertz from the flexible use C-band deployment so it is presumed that most of the burden should fall on the aviation community to filter their devices appropriately,” Farrar told Fierce via email. “However, the issue has now been widened to a question of spurious emissions into the 4.2-4.4GHz band and that may need more careful examination.”

A major aspect of the August presentation by aviation stakeholders centered on the need for carriers deploying the first tranche of C-band to implement short-term steps – with the FCC and FAA stepping in to facilitate.

Some of those steps include lowering base station and user equipment maximum spurious emission limits across the 4.2-4.4 GHz band to protect fixed-wing airplanes (non-commercial air transport) and helicopters, and protection areas with no base stations in line with runways for commercial airplanes.

They cited a variety of reasons efforts by the airline and aerospace entities aren’t doable near-term and need to be complemented by 3.7 GHz licensee solutions. According to ex parte filings, including one on August 10, the groups that presented to the FCC explained why efforts on the part of airlines and aerospace companies alone won’t be able to close what they call “a mitigation gap” – meaning between the time carriers start deploying C-band in the first 46 partial economic areas in December and until longer-term measures can be put in place by the aviation community.

For example, the option to add band pass filters to certified aircraft before 3.7 GHz deployments start in major markets “is a practical impossibility,” the group said, “and fails to offer a comprehensive solution to mitigate the risks of interference to radio altimeters.”

Retrofitting with out-of-band filter installations also" will do nothing to address the risk of interference from potential flexible use spurious emissions" into the 4.2-4.4 GHz band, the groups said, which could potentially impact aircraft like business aviation, regional transport airplanes and helicopters.

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Measures, such as new aviation standards and related certified equipment that is then installed on aircraft “will take a number of years,” the groups said.

Representatives including the Airborne Public Safety Association, Airbus, Boeing, Lockheed Martin and others, expressed that “the aviation and aerospace industry has determined in good faith that closing the mitigation gap is not possible without the flexible use licensees being required to take their part. Consequently, without appropriate mitigation measures taken by 3.7 GHz flexible use licensees to reduce sufficiently the potential for harmful interference to radio altimeters, the result is likely to be substantial disruption to the use of the National Airspace System. This will adversely impact the flying public, the economy, and critical aviation services.”

As one example, the representatives pointed to air ambulances arriving at a heliport in a large medical center.

“Analysis shows that flexible use base stations, that implement no mitigations to protect radio altimeters from harmful interference, have the potential to wreak havoc on the use of heliports at hospitals and in medical centers, as well as at the countless random offsite locations where helicopters frequently land in first responder situations,” the ex parte states.